Filament pre-warm resistor

Jul 02, 2016 1 Replies
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Playing around with a Numitron tube, driving the filaments from a uP outputs.



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Most lamp driver IC datasheets recommend a filament pre-warm resistor to reduce "thermal shock." But these tubes are common anode and are driven by pulling the cathodes low. Is there a way to implement in this configuration?



Second question: I'm driving the anode with an eBay special 0.5- something volt input to volt to 5 volt boost converter from a LiPo battery. This module has a "shutdown" pin which will, obviously, shut the converter down when pulled low.



Would it be possible to implement PWM dimming of all segments by driving the converter shutdown pin from a uP output?


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T

The Numitron tubes (and the Russian equivalents) do not have anodes or cathodes. There are just terminals to internal filaments of the tube. If you want to pre-warm the tubes, you need open-collector/open-drain drivers, and you can parallel a resistor from each driver output to ground to keep the filaments warm.

You'll get plenty of thermal transients to the filaments, if you want to dim using PWM.

It depends on the speed of the converter shutdown/startup if there is any sense to dim the displays switching the converter on and off. The speed must be more than the flicker fusion frequency of the observer eye, and you must avoid any interference effects with possible display multiplexing frequency.

-TV
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